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Merged
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May 16, 2025
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langref updates for aarch64 trampoline #139740

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langref updates for aarch64 trampoline
vtjnash May 13, 2025
34d45bc
Update LangRef.rst
vtjnash May 14, 2025
04193da
Update LangRef.rst
vtjnash May 14, 2025
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17 changes: 11 additions & 6 deletions llvm/docs/LangRef.rst
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Expand Up @@ -410,8 +410,8 @@ added in the future:
calling convention: on most platforms, they are not preserved and need to
be saved by the caller, but on Windows, xmm6-xmm15 are preserved.

- On AArch64 the callee preserve all general purpose registers, except X0-X8
and X16-X18.
- On AArch64 the callee preserve all general purpose registers, except
X0-X8 and X16-X18. Not allowed with ``nest``.

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Please don't put "may crash llvm". That's not contractual: it's a bug if we crash instead of printing a proper error message.

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I'd also consider it a bug and a crash if LLVM prints any error messages (llvm_unreachable is supposed to be, um, unreachable in code that passes the verifier, but it is used for trampoline in backends such as X86ISelLowering with the message "Unsupported calling convention"). I guess it is instead currently just an undocumented behavior that calling conventions are allowed to print errors if the backend doesn't support it?

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We don't usually add a "not all backends support this construct" disclaimers to LangRef; it would pop up in a lot of places, and it wouldn't really be helpful in most of those places.

There is a way to print a proper diagnostic from the backend without crashing, we just don't use it in all the places we should.

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Okay, I see LLVMContext->emitError now (which defaults to calling exit(1), but which can be customized). I guess that existence could potentially be better communicated to frontends? I find it curious that the entire section on how to handle errors in llvm makes no reference of this: https://llvm.org/docs/ProgrammersManual.html#recoverable-errors. And a recently discourse thread on how to handle errors in the backend also claimed it wasn't possible because the proper way to handle it without crashing doesn't exist: https://discourse.llvm.org/t/rfc-report-fatal-error-and-the-default-value-of-gencrashdialog/73587

Would you be willing to review and merge a PR to that section of the manual if I were to add a mention there?

(and perhaps we should also delete the exit(1) in diagnose, instead providing that behavior as an option, so that backends need to properly test that function not returning, and that front end tools are forced to actually implement it better using whatever error recovery–or exit(1) call–that they have)

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I assumed everyone was aware of LLVMContext::diagnose... I did mention it in the discussion (although not by name).

For documentation, that doesn't seem like it's quite the right section of the manual; it's describing generic "error handling", not the rules specific to LLVM passes. We probably should have a description somewhere, though.

I think the reason diagnose() exits by default is so you don't accidentally forget to write a handler, and then continue processing bogus output. In practice, you almost always want one.

The idea behind this convention is to support calls to runtime functions
that have a hot path and a cold path. The hot path is usually a small piece
Expand Down Expand Up @@ -447,9 +447,9 @@ added in the future:
R11. R11 can be used as a scratch register. Furthermore it also preserves
all floating-point registers (XMMs/YMMs).

- On AArch64 the callee preserve all general purpose registers, except X0-X8
and X16-X18. Furthermore it also preserves lower 128 bits of V8-V31 SIMD -
floating point registers.
- On AArch64 the callee preserve all general purpose registers, except
X0-X8 and X16-X18. Furthermore it also preserves lower 128 bits of V8-V31
SIMD floating point registers. Not allowed with ``nest``.

The idea behind this convention is to support calls to runtime functions
that don't need to call out to any other functions.
Expand Down Expand Up @@ -21120,7 +21120,12 @@ sufficiently aligned block of memory; this memory is written to by the
intrinsic. Note that the size and the alignment are target-specific -
LLVM currently provides no portable way of determining them, so a
front-end that generates this intrinsic needs to have some
target-specific knowledge. The ``func`` argument must hold a function.
target-specific knowledge.

The ``func`` argument must be a constant (potentially bitcasted) pointer to a
function declaration or definition, since the calling convention may affect the
content of the trampoline that is created.


Semantics:
""""""""""
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